1. Field
Example embodiments relate to magnetic memory devices and/or methods of writing data to the magnetic memory devices.
2. Description of the Related Art
A magnetic random access memory (MRAM) is a memory device that stores information by using a change in resistance of a magnetic tunneling junction (MTJ) element. The resistance of the MTJ varies depending on a magnetization direction of a free layer. When the magnetization direction in the free layer is the same as that of a pinned layer, the MTJ element has a low resistance value. Conversely, when the magnetization direction of the free layer is opposite to that of the pinned layer, the MTJ element has a high resistance value. A low resistance state and a high low resistance state of the MTJ element correspond to data ‘0’ and ‘1’, respectively. Due to its non-volatility, high speed, and high endurance, the MRAM has emerged as one of the most promising next-generation non-volatile memory devices.
Early stage magnetic memory devices employ Oersted fields to switch a MTJ device, and create an Oersted field by using a conducting wire in which current flows. For example, a magnetic field generated by current flowing through a bit line and a word line is used to switch the magnetization direction of a free layer. Furthermore, a switching method called toggle switching may significantly reduce writing errors. Toggle switching refers to a process in which current is applied to a bit line and a word line with a time delay therebetween according to a predetermined (or, set) order. In this process, to reverse the magnetization direction of the free layer, the order in which current is applied to two conducting wires may be reversed instead of changing the direction of current.
However, when the size of a unit cell is reduced to implement a high density memory device, a switching field increases with an increase of the coercivity of a free layer, thereby resulting in an increase in amplitude of applied current. Furthermore, in a memory cell array having a plurality of memory cells, switching may occur even in a free layer in an inappropriate cell. Thus, a magnetic memory device employing switching induced by Oersted fields may have difficulty in achieving selectivity and high density.
To overcome the above problems, a spin transfer torque magnetic random access memory (STT-MRAM) which switches the magnetization direction of a memory cell by using a current-induced STT is receiving much attention as a next-generation high density memory device. However, STT-MRAM does not allow toggle switching, and providing a margin for reducing writing errors becomes a key issue for the STT-MRAM.